1. Field of the Invention
The present invention relates to a node device, and in particular to a node device composing a ring network connected to another network.
Recently, a traffic increase in an Ethernet (registered trademark) and the IP (Internet Protocol) packet-based Internet, a rapid increase of ADSL (Asynchronous Digital Subscriber Line) users and a rise of a VoIP (Voice over IP) technology have been remarkable. Thus, instead of a ring network using a conventional SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) device, a packet ring network (hereinafter, occasionally and simply referred to as ring) which can directly process the packets on the ring and enables a bandwidth management of the packets and a statistical multiplexing effect is noticed. When such a packet ring network is connected to another network, it is required to connect the packet ring network to the other network with a plurality of node devices (hereinafter, referred to as redundant node devices) for enhancing a reliability of the network.
2. Description of the Related Art
FIG. 19 shows a network composed of a backbone network 210, a ring network 200a and a tree network 220. The backbone network 210 is composed of node devices 100z_11-100z_14, 100_15 and 100_16. The ring network 200a is composed of node devices 100z_21, 100z_22 and 100_23-100_27. The tree network 220 is composed of node devices 100z_31, 100z_32, 100_33-100_37. The redundant node devices 100z_11, 100z_12 and the redundant node devices 100z_13, 100z_14 (occasionally, represented by a reference numeral 100z; the node devices other than the redundant node devices are occasionally represented by a reference numeral 100 and occasionally referred to as non-redundant node devices) of the backbone network 210 are respectively connected to the redundant node devices 100z_22, 100z_21 of the ring network 200a and the redundant node devices 100z_32, 100z_31 of the tree network 220, whereby the backbone network 210 is connected to the ring network 200a and the tree network 220.
When a redundant network is composed by adding a redundant node device, numerous fibers are required for newly connecting the non-redundant node devices 100 to an added redundant node device 100z, in the tree network 220 and a mesh network (not shown). However, there is an advantage that only fibers for connecting adjoining node devices are required in the ring networks 200a and 210.
Also, when a node redundancy system is realized in the tree network 220 and the mesh network of a packet system like the Ethernet, a Spanning Tree Protocol (STP) is generally used. In the STP, a switchover to a detour route upon occurrence of a line fault or a node device fault requires almost one minute. Recently, a Rapid Spanning Tree Protocol (RSTP) that is an improved STP has been reviewed, which also requires several seconds of a switchover time.
In the node redundancy by this STP system or the like, a demand of a communication carrier or the like for i.e. equal to or less than 50 ms of a switchover time can not be satisfied, so that a node redundancy system with a higher reliability has not been provided.
On the other hand, as a ring protocol used in the ring network, there is an RPR (Resilient Packet Ring) protocol which brings about effects of statistical multiplexing and bandwidth reusing by directly processing the packets on the ring, and which realizes a fault recovering function (protection function) by a high speed switchover for equal to or less than 50 ms. For the RPR of the packet ring, standardization efforts have been pursued as IEEE802.17RPR. Together with this standardization, it is expected that the demand for the packet ring network noticed in a metro area grows more and more. It is to be noted that the RPR represents a recent packet ring, and is distinguished from a conventional token ring and an FDDI ring.
A node device and a redundant node device according to the present invention described later can be applied to the RPR. Therefore, a basic arrangement of a network, a topology table and a packet in the RPR will now be described by referring to FIGS. 20A, 20B and FIG. 21.
FIG. 20A shows an arrangement of a general RPR ring network. This network is a bidirectional double ring network (hereinafter, occasionally and simply referred to as ring) in which node devices 100a_11-100a_16 (occasionally, represented by a reference numeral 100a) supporting the RPR are connected in this order with outer and inner rings. It is to be noted that an RPR network 200a is not connected to another network in FIG. 20A, so that a node device corresponding to the redundant node device 100z is not arranged.
FIG. 20B shows an arrangement of a ring topology table 70a held in the node device 100a_11. This table 70a is composed of a node device address 71, an east hop number 74, a west hop number 75, an east route selection 76 and a west route selection 77. An address of each node device 100a is registered (an address identical to the reference numeral is registered in FIG. 20B) in the node device address 71. The number of hops to the position of another node device 100a starting from the node device 100a_11 in an east direction is registered in the east hop number 74. It is indicated that e.g. the node device 100a_13 is in the position of the hop number=“2” from the node device 100a_11 in the east direction. The number of hops in a west direction is registered in the west hop number 75. It is indicated that e.g. the node device 100a_13 is in the position of the hop number=“4” from the node device 100a_11 in the west direction.
The east route selection 76 and the west route selection 77 indicate from which of the east side, the west side or both sides the node device 100a transmits a packet addressed to another node device 100a. The packet is transmitted from the side where “ON” is set. For example, the packet addressed to the node device 100a_13 is transmitted from the outer ring on the east side since the east route selection=“ON” and the west route selection=“OFF”. Thus, the packet reaches the node device 100a_13 with the hop number=“2”, which is fewer than the case of transmission from the inner ring on the west side by a hop number of “4”−“2”=“2”.
It is to be noted that “ON” and “OFF” of the east route selection 76 and the west route selection 77 are changed by a ring topology which changes by a fault of the route (link) and the node device. Namely, “ON” and “OFF” of the route selections 76 and 77 are determined so as to select an optimum route for distributing the packet at that time.
It is to be noted that while a topology table of another node device 100a is the same as that of the table 70a, only the values set in the east hop number 74, the west hop number 75, the east route selection 76 and the west route selection 77 are different from those in the table 70a. 
FIG. 21 shows a format of a general RPR packet 700, that is composed of a Time To Live (TTL) 710, a Base_Ring_Control 720, a destination address 730, a source address 740, a TTL_Base 750, an Ex_Ring_Control 760, a Header_CRC 770, a protocol data unit (PDU) 780 and FCS 790. Since the detail of these fields is a basically known matter of the RPR, the description thereof will be herein omitted.
It is to be noted that the protection function is mounted on the RPR itself. Therefore, the RPR can notify a fault to each node device on the ring by transferring a protection message on an RPR layer (MAC sub-layer) upon occurrence of a line fault, a node device fault or the like.
Also, in the double ring arrangement of the RPR, a high-speed switchover to a detour route can be performed by switching over a ring which transmits the packets upon occurrence of the line fault or the like.
The characteristics of the RPR will now be summarized including the above-mentioned characteristics (see non-patent document 1):    (1) Bidirectional double ring network is supported;    (2) MAC (Media Access Control) layer (layer 2) is supported;    (3) Effective utilization ratio of bandwidth is high;    (4) Plug & Play is supported;    (5) Fault switchover time is equal to or less than 50 ms.[Non-patent document 1] IEEE802.17 draft V2.1
However, in the IEEE802.17 RPR, a function concerning a node redundancy system which enables an arrangement of a working node device and a protection node device on the ring is not prescribed, and no addition of a node redundancy function to the IEEE802.17 RPR protocol itself is expected. Also, in an RFC2892 (The Cisco SRP (Spatial Reuse Protocol) MAC Layer Protocol) of the IETF which forms the basis of the IEEE802.17 RPR, an effective utilization of the bandwidth and the redundancy of the ring are enabled. However, nothing is prescribed therein for the node redundancy system in the ring network.